1. Field of the Invention
The present invention relates to sensors for measuring the pressure of fluids, and particularly to pressure sensors that involve the use of a diaphragm that is deflected by a pressure differential. More particularly, the invention relates to the use of multiple diaphragms to make a single pressure sensor with advantageous characteristics.
2. Technical Background
There exists a great need for the ability to precisely measure pressures of fluids (liquids or gases), particularly where there is demand for very high overpressure capability relative to the pressure range of interest or in other conditions that are typically harsh for pressure sensors. This capability has a large number of diverse applications, particularly those that involve differential pressure sensing in systems capable of withstanding high pressures. These applications may include monitoring of fluid control valves for flow inventory systems (as may be found, for example, in automated fire suppression systems and similar water distribution systems, nuclear reactors, refineries and similar chemical process plants, and utility systems), for the improved control of combustion engines, and in other critical pressurized liquid and gas flow systems.
Diaphragm-type pressure sensors are those pressure sensors that transduce pressures into electrical signals by means of a flexible diaphragm over an opening bored or etched out of a substrate. The diaphragm may be affixed over the opening, may be formed by a thinning of the substrate layer in a region, or may be created by any number of other methods known to those skilled in the art. The diaphragm of these type of sensors deforms or flexes to a degree determined by the differential pressure between the two sides of the diaphragm; on one side of the diaphragm exists the one pressure region of interest, and on the other side of the diaphragm exists another pressure region of interest or of some known reference pressure. The word “opening” as used in this patent is meant to signify any space into which or out of which a flexible diaphragm may deflect. An opening may be a full bore completely through a substrate or may consist of an evacuation underneath a diaphragm, e.g. unpressurized, pressurized or a vacuum.
Pressure-sensing diaphragms may behave as thin plates or thick plates, depending upon the diaphragm's material and its dimensions. When a thin-plate diaphragm deflects under a pressure difference, all areas on the surface of the diaphragm that is on the outside of the arc of deflection experience tension. When a thick-plate diaphragm deflects under a pressure difference, the areas of the diaphragm on the inside of the arc of deflection undergo compression and the areas of the diaphragm on the outside of the arc of deflection experience tension.
The strain or deflection of a pressure-sensing diaphragm may be transduced into an electrical signal. The two most typical approaches involve, respectively, resistive transducer elements and capacitive transducer elements.
The most common form of the resistive approach involves one or more strain gauges bonded to or diffused into the diaphragm. With a pressure difference between its two sides, the strain induced on an area of the diaphragm due to its deflection causes the one or more strain gauges to change dimensions in a way that depends on their placement position and orientation on the diaphragm, the direction of the diaphragm deflection, and whether the diaphragm is a thick-film or thin-film diaphragm. This change of dimensions involves an expansion in one or two dimensions and a contraction in the other dimensions, and has the effect of increasing or decreasing the strain gauges' resistive values. The change in resistance of the strain gauges can be measured electrically by using, for example, a circuit such as a Wheatstone bridge. When resistive transducer elements are used, at least one transducer element is placed in one leg of such a bridge, making the bridge a quarter bridge; more preferably, at least two transducer elements are placed in two legs of such a bridge, making the bridge a half bridge; even more preferably, at least four transducer elements are placed in four legs of such a bridge, making the bridge a full bridge. While the full bridge configuration is preferred for its improved sensitivity and linearity of output signal to applied pressure on the sensor, the half bridge configuration is used where physical conditions do not allow mounting of two complementary pairs of transducer elements to a diaphragm, and the quarter bridge configuration is used where it is impossible to mount even a single pair of transducer elements to a diaphragm. The signal output of the bridge may be conditioned (e.g., filtered, amplified, digitized, etc.) before the electrically-transduced pressure signal is implemented as part of a larger system; signal conditioning circuitry may be fabricated on the same substrate as that upon which the diaphragm is mounted, or signal conditioning circuitry may be manufactured separately and later connected to the pressure sensor.
In the most common form of the capacitive approach, the pressure diaphragm itself constitutes one plate of a capacitor that changes its value under pressure-induced displacement. In such a sensor both plates of the capacitive sensor may be deformable. As the capacitance of a parallel-plate capacitor is inversely proportional to the distance between plates, the capacitance value of such a diaphragm-type pressure sensor increases as the diaphragm deflects inward toward the other plate of the capacitor, and, conversely, the capacitance value of such a diaphragm-type pressure sensor decreases as the diaphragm deflects outward out of its substrate away from the other plate of the capacitor. The simplest methods for measuring capacitance value involve charging and discharging the capacitor under test with a known current and measuring the rate of rise of the resulting voltage; the slower the rate of rise, the larger the capacitance. Bridge circuits operating under alternating current conditions can be used to measure capacitances as they can with resistances. The signal output of the capacitance measurement circuit may be conditioned before the electrically-transduced pressure signal is implemented as part of a larger system, and signal conditioning circuitry may be fabricated on the same substrate as that upon which the diaphragm is mounted, or may be separately manufactured and attached.
Under typical manufacturing processes, the diaphragm, its substrate, and any accompanying signal conditioning electronics are trimmed into a single die, which is then optionally combined with other circuitry or mounted into a partially sealed housing that leaves the diaphragm exposed, which then may be installed into the larger system in which pressures of interest are to be measured. The dimensions, area, thickness, and material properties (such as stiffness) of the diaphragm determine its maximum operating pressure, maximum recovery pressure, and its burst pressure.
The maximum operating pressure is the pressure that can be placed across the diaphragm up to which the sensor produces an accurate reading. The maximum operating pressure determines upper limit of the sensor's useful range.
The maximum recovery pressure, or overpressure limit, is the pressure that can be placed across the diaphragm without damaging the pressure sensor. A diaphragm-type sensor is capable of returning to normal operation up to its maximum recovery pressure, even though it may deliver a saturated (i.e., constant, inaccurately low) output signal in the range between its maximum operating pressure and its maximum recovery pressure, such range being known as “overpressure.” Once the sensor exceeds its overpressure limit, the sensor may not be capable of returning to normal operation owing to a permanent deformation of the diaphragm or other structural or electrical damage resulting from the strain of the diaphragm beyond its elastic limits. Hence, a diaphragm-type sensor that has sustained damage from overpressure may report output even with no load on the sensor, i.e., no substantial elastic deflection of the diaphragm. Commercially available harsh-environment diaphragm-type pressure sensors are typically rated for a maximum recovery pressure of about 50% over maximum operating pressure.
The burst pressure of a diaphragm-type sensor is the amount of pressure difference that can be placed across the diaphragm before it suffers a mechanical failure that results in leakage of fluid across the barrier created by the diaphragm. Commercially available harsh-environment diaphragm-type pressure sensors are typically rated to several hundred or several thousand psi, for a burst pressure to maximum operating pressure ratio (also known as the “burst pressure ratio”) of less than 3:1.
The trade-off between diaphragm characteristics, e.g. opening size, thickness and stiffness, and burst pressure ratio is always a difficult challenge in the case of sensors that require high sensitivity in environments that may also present high maximum pressures. A diaphragm-type sensor can be made to have a higher sensitivity by using a larger, thinner, more pliant diaphragm, as a greater range of mechanical deflection under strain is more easily transduced into a greater range of electrical signal output, but correspondingly these diaphragms are also more susceptible to fracturing under overpressure conditions, destroying the sensor.
It is therefore the object of the present invention to provide an improved sensor with increased sensitivity along with a corresponding higher maximum recovery pressure and higher burst pressure ratio.